In today's cellular systems the use of multiple antennas (MIMO) for transmission is becoming increasingly important. The antenna system can be designed with either correlated or uncorrelated antenna elements, or combinations thereof where some antenna elements are correlated and others uncorrelated.
To fully be able to exploit the potential of these MIMO systems the correlated antenna elements need to be calibrated both in phase and amplitude. Antenna calibration is mostly important when an antenna configuration includes correlated antenna elements and for features that require well directed beams. An example of such an antenna configuration is the commonly used 4-antenna configuration with 2 closely spaced cross-polarized elements 102a,102b, which is illustrated in FIG. 1. The cross-polarized elements, or cross-poles, 102a,102b, each comprises two mutually uncorrelated cross-polarized antenna elements, 1+3 (102a) and 2+4 (102b), respectively. The antenna elements 1 and 2 are illustrated as dashed lines, and the antenna elements 3 and 4 are illustrated as solid lines in FIG. 1. The “dashed” antenna elements, 1 and 2 have the same polarization and compose a first pair of correlated antennas. The “solid” antennas, 3 and 4 have the same polarization and compose a second pair of correlated antennas.
One source of error is timing differences between antenna branches due to feeder length differences or delay differences in the radio chains. Most calibration techniques require additional hardware or other adjustments for calibration purpose only, which is expensive and inefficient.